1. Field of the Invention
The present invention relates to solar power sources, their regulators and in particular to a circuit to protect against arcing on solar panels or within the solar array drive mechanism (SADM) of a spacecraft.
2. Description of the Prior Art
Solar array panels of a geostationary telecommunication satellite are maintained in a sunlight configuration by a solar array drive mechanism which rotates the array wings once every 24 hours. In order to transfer the power generated by the solar cells to the satellite on-board electrical system, a slip ring and pick-off brush assembly is incorporated within the solar array drive mechanism. Such a slip ring comprises concentrical conductor rings separated by insulating barriers.
A classical power system of a spacecraft comprises a power regulator for regulating the voltage of the main power bus of the spacecraft. This regulator includes several power dump stages which are used to shunt the excess of solar array current in order to generate a regulated voltage. A power dump stage is disclosed in the U.S. Pat. No. 4,186,336. A functionally equivalent circuit using a more recent technology is shown in FIG. 1. This stage is either in a “Dump ON” or in a “Dump OFF” state controlled by a “DoD” (Dump on Drive) signal. The power dump stage 3 comprises a MOSFET transistor M10 connected between the solar array line and the ground, which shunts the solar array voltage when it is set in its ON state (corresponding to the Dump On state of the stage) by the DoD signal. This stage further comprises two serial blocking diodes D1, D2 inserted in the line linking the solar array and the main power bus, so as to prevent current to flow back from the power bus to the power dump stage or the solar array.
Over recent years, satellite power bus voltages have increased and several cases of power losses have been observed in satellite on-board electrical systems, resulting in a significant degradation of the spacecraft performances. These power losses have been attributed to damage caused by sustained voltage arcing occurring either on a solar array panel or even within the solar array drive mechanism causing the power loss of a full solar array wing. The initiating mechanism of this voltage arcing is usually attributed to some form of contamination. If metallic particles bridge across two electrical conductors (solar cells, slip rings) having a significant potential difference, the initial current flow will likely evaporate the metallic particle. Since classical spacecraft shunt regulators usually comprise two serial blocking diodes (D1, D2) between the solar array and the main power bus, if an arcing event occurs that demands more current than the solar cells can provide, the solar array section voltage will collapse.
After the fusing of the particle, the arc may extinguish, but if sufficient plasma still exists and localized damage has occurred, a sustained arcing event can be initiated. If the latter situation proves to be the case, the extreme heat generated by the arcing event will rapidly degrade any local insulation barrier and quickly results in a permanent short circuit condition, resulting in a permanent power loss for the spacecraft.
The initial arcing potential can be low but as the material of the contact points are eroded away, the arc potential will typically increase.
Although such an arcing event is known to be a rare occurrence, it is essential to prevent such arcing events in order to maintain the performance and required life span of satellites.